Free piston Stirling cryocoolers and other free piston Stirling heat pumps are typically powered by a linear, electric drive motor which drives the free piston in reciprocation. The rate at which heat is pumped by the Stirling cooler is an increasing, continuous function of the displacement at which its piston is driven. Consequently, it is desirable to control the piston displacement by controlling the drive voltage applied to the motor which drives the piston. It is desirable to control the piston displacement as a function of the temperature of the refrigerated compartment in order to stabilize the temperature within the design limits and to avoid piston-displacer collision resulting from reduced loading before the nominal design temperature is reached, especially initially when the refrigerated compartment temperature is near the ambient temperature.
More specifically, when the cold end temperature is above the selected design value, it is desirable that the piston displacement be increased in order to increase the thermal energy pumping rate. When the cold end temperature is below the design value, it is desirable to reduce the drive voltage applied to a linear motor in order to reduce displacement and thereby reduce the thermal pumping rate.
It is therefore an object of the present invention to provide a controlled drive circuit for driving the linear motor with a controllable AC voltage at the fundamental frequency equal to the operating frequency which ordinarily is the resonant frequency of the motor and its load. The drive voltage can be a function of cold end temperature and, if desired, other control variables, such as pressure and time, in order to control the stroke or displacement of the piston, both during cool-down from ambient temperature, during which time the dynamics of the sealed, free piston, Stirling cryocooler change because of changing pressure and temperature of the working gas and also in order to stabilize the cold end temperature after operating temperature has been reached.
U.S. Pat. No. 3,220,201 discloses a control circuit in which the width of a rectangular pulse, at the fundamental operating frequency of the drive motor, is controlled in order to maintain a constant fundamental piston amplitude under all conditions. The drive system of this patent not only does not control the free piston displacement as a function of temperature, but, more importantly, it results in an intermediate off time between the drive pulses.
The significant difficulty which that causes arises from the fact that the drive motor must be driven by a power switching circuit consisting of power switching transistors, connected in a switching configuration, such as a conventional H-bridge. The switching technique described in that patent requires an intermediate interval between the pulses when all the switching transistors are turned off. Because the drive voltage is driving a load which includes a significant inductive reactance, the current cannot be instantaneously switched off. Consequently, an attempt to switch all the power switching transistors to an off state causes the instantaneous voltage amplitude applied to the motor to become poorly controlled and distorted. This consequently produces non-sinusoidal motor current.
It is an object of the present invention to control the voltage applied to the motor in a manner so that no instant of time occurs during which all power switching transistors are turned off. It is a purpose and feature of the present invention to provide a circuit in which a current path always exists through the power switching circuit and the motor and a drive voltage is always applied to the motor.
Other cryogenic cooler control systems are shown in U.S. Pat. Nos. 3,991,586 and 4,417,448.
It is therefore a further object and feature of the present invention to provide a circuit which is capable of applying a continuously variable drive voltage to the piston drive motor at the operating frequency, the drive voltage being a substantially linear function of a control variable, such as cryocooler temperature, so that the drive motor displacement can be continuously varied over the range of the control variable.